JSFiddle - React, Tailwind, and code Playground

HTML

<span class="extended">I'm extended</span>
<span>I'm  not extended</span>

JavaScript

function test() {
    this.style.fontWeight = "bold";
}

window.onload = function(){
    var elements = document.getElementsByClassName('extended');
    for (var i = 0; i< elements.length;i++){        
        elements[i].test = test;
    }
    var spans = document.getElementsByTagName("span");
    alert("span[0].test: " + typeof spans[0].test
          +"\nspan[1].test: " + typeof spans[1].test); 
    spans[0].test();
};